The present invention relates to a sheet feeding apparatus for sheet-shaped paper used in apparatuses such as a copying machine, a facsimile machine and a printer, and relates to a sheet feeding apparatus and an automatic document feeding apparatus that feeds a document both used in an automatic two side image forming unit (ADU) which forms images on both sides of a sheet-shaped paper.
An image forming apparatus such as a copying machine, a facsimile or a printer has so far been provided with a sheet feeding apparatus which takes out documents or sheet-shaped papers stacked on a document tray, a sheet-feeding cassette or a sheet stacker one sheet by one sheet from the uppermost sheet, and feeds it to an image forming section.
As a sheet feeding apparatus wherein an operation to load sheets in a sheet feeding cassette has been improved, a sheet feeding apparatus called as that of a friction separation roller system has generally been put to practical use. A sheet feeding apparatus of this friction separation roller system is also mounted on the automatic two side image forming unit (ADU).
The sheet feeding apparatus of this system is disclosed in TOKKAISHO Nos. 62-105834 and 63-57447.
FIG. 9(a) is a front section of a conventional sheet feeding apparatus, and FIG. 9(b) shows a sectional view taken on line A--A.
In the sheet feeding apparatus, at the separation sheet feeding section where feed roller 2 and reverse roller 3 are provided to be pressure contact with each other at the downstream side in sheet feeding made by pickup roller 1 which feeds out plural sheets P, the sheets P thus fed out are caused to be inserted between the feed roller 2 and the reverse roller 3 to be reversed so that the sheets may be fed one sheet by one sheet.
A leading edge in the sheet feeding direction on sheets P stacked on movable bottom plate 5 in sheet housing holder main body (sheet feeding cassette main body or intermediate stacker main body) 4 of a sheet housing means (sheet feeding cassette or intermediate stacker section) is arranged so that the leading edge is brought into pressure contact with the pickup roller (first sheet feeding roller) 1 by the upward movement of bottom plate lifting lever 6, and the sheet P is fed out by the rotation of the pickup roller 1.
Following the pickup roller 1, there is provided a sheet reversing mechanism for preventing double feeding wherein feed roller 2 and reverse roller 3 are provided to face each other, and a mechanism wherein in the case of double feeding (feeding of plural sheets) of sheets P, the uppermost sheet P only among sheets of double feeding is fed out and other sheets are stopped or returned, is provided.
In the sheet feeding course of this sheet feeding apparatus, the leading edge of sheet P is first lifted by the upward movement of the movable bottom plate 5, and it stops at the position where the upper face of sheet P comes in pressure contact with an outer circumferential surface of the pickup roller 1. Then, a sheet feeding signal makes the pickup roller 1 and the feed roller 2 to start rotating. The reverse roller 3 is being driven to rotate in the direction opposite to the advance direction of sheet P and is urged by a spring to be in pressure contact with the feed roller 2.
When the reverse roller 3 is in direct contact with the feed roller 2 (when there is no sheet in the nip), or when one sheet P is fed into the nip, the limit torque for the reverse roller 3 is exceeded to cause a slip and the reverse roller 3 is driven to rotate by the feed roller 2. However, when two or more sheets P are fed into the nip, the limit torque exceeds the friction force between sheets and causes the feed roller 2 to rotate reversely to push back the lower sheet, whereby double feeding (feeding plural sheets) can be prevented. Sheet P thus reversed and separated to be one sheet through the above-mentioned manner is conveyed to unillustrated registration roller 28 (second sheet feeding roller).
The sheet feeding apparatus mentioned above still has the following problems.
(1) A contact portion where bottom surface 5A of the movable bottom plate 5 comes in pressure contact with tip portion 6A of the bottom plate lifting lever 6 has been of a line contact or a surface contact. It is difficult to accurately maintain the parallelism between hinge portion 5B of the movable bottom plate 5 and rotary shaft 7 of the bottom plate lifting lever 6. It is further difficult to precisely maintain manufacturing accuracy for flatness of the bottom surface 5A of the movable bottom plate 5 which is a contact portion where the tip portion 6A of the bottom plate lifting lever 6 is brought into pressure contact by the upward driving of the tip portion 6A of the bottom plate lifting lever 6 and for linearity of the tip portion 6A of the bottom plate lifting lever 6. Therefore, the tip portion 6A of the bottom plate lifting lever 6 for upward driving and the bottom surface 5A of the movable bottom plate 5 can not be in line contact each other, resulting in one side contact. When sheet P on the movable bottom plate 5 which is in the one side contact is brought into pressure contact with an outer circumferential surface of the pickup roller 1, the sheet P on the movable bottom plate 5 is not brought into pressure contact evenly with plural pickup rollers 1 arranged on rotary shaft 8 of the pickup rollers 1, and sheet feeding troubles such as skewed feeding and biased feeding are caused in sheet feeding.
(2) There is a space between sheet stopper surface 4A located inside the sheet housing holder main body 4 which is hit by the leading edge of sheet P housed in the sheet housing holder main body (sheet feeding cassette main body or intermediate stacker main body) 4 and a rise and fall locus of the edge portion of the movable bottom plate 5. The distance of this space in the sheet feeding direction is smallest when the movable bottom plate 5 is at its lowest position, and is largest when the movable bottom plate 5 is at its highest position. Therefore, when the leading edge of sheet P fed out of the movable bottom plate 5 at the upper most position, especially of a sheet curled downward or of a thin sheet, is conveyed to guide member 9 located at the downstream side of sheet feeding, the leading edge is caught by a part of the sheet stopper surface 4A having the space, resulting in occurrence of sheet feeding troubles.